We elaborate on a recently suggested effective Lagrangian for charged-current and neutral-current electroweak interactions which in comparison with the standard electroweak theory contains three free parameters Delta x, Delta y, epsilon which quantify different sources for violations of SU(2) symmetry. Within the standard SU(2)(I) x U(1)(Y) electroweak theory, we present both exact and very much refined approximate analytical one-loop expressions for these parameters in terms of the canonical input, G mu, M(Z), alpha(M(Z)(2)), the top-quark mass, m(t), and the Higgs-boson mass, M(H). We re-emphasize the importance of discriminating between the empirically well-known purely fermionic (vacuum polarization) contributions to Delta x, Delta y, epsilon and the empirically unknown bosonic ones with respect to present and future electroweak precision tests. The parameters Delta x and epsilon are hardly affected by standard bosonic corrections, while the full one-loop results for Delta y differ appreciably from the ones obtained by taking into account fermion loops only. A detailed comparison with the experimental data on M(W)+/-/M(Z), S-W(-2), Gamma e shows that these data start to become accurate enough to be sensitive to standard (bosonic) contributions to Delta y beyond fermion loops.